1. Field of the Invention
The present invention relates to a method for fabricating a circuit device, and in particular, a method for fabricating a thin type circuit device, using two conductive layers, which is able to achieve multi-layered connection.
2. Description of the Prior Arts
Recently, IC packages have been actively employed in portable devices, and small-sized and high density assembly devices. Conventional IC packages and assembly concepts tend to greatly change. For example, this is described in, for example, Japanese Laid-Open Patent Publication No. 2000-133678. This pertains to a technology regarding a semiconductor apparatus in which a polyimide resin sheet being a flexible sheet is employed as one example of insulation resin sheets.
FIG. 13 through FIGS. 15A,15B and 15C show a case where a flexible sheet 50 is employed as an interposer substrate. Also, the views illustrated upside of the respective drawings are plan views, and the views illustrated downside thereof are longitudinally sectional views taken along the lines A—A of the respective drawings.
First, copper foil patterns 51 are prepared to be adhered to each other via an adhesive resin on the flexible sheet 50 illustrated in FIG. 13. These copper foil patterns 51 have different patterns, depending upon cases where a semiconductor element to be assembled is a transistor or an IC. Generally speaking, a bonding pad 51A and an island 51B are formed. Also, an opening 52 is provided to take out an electrode from the rear side of the flexible sheet 50, from which the above-described copper foil pattern 51 is exposed.
Subsequently, the flexible sheet 50 is transferred onto a die bonder, and as shown in FIG. 14, a semiconductor element 53 is assembled or mounted. After that, the flexible sheet 50 is transferred onto a wire bonder, wherein the bonding pads 51A are electrically connected to the pads of the semiconductor elements 53 by thin metal wires 54.
Finally, as shown in FIG. 15A, sealing resin 55 is provided on the surface of the flexible sheet 50, and the surface thereof is completely sealed with the sealing resin 55. Herein, the bonding pads 51A, island 51B, semiconductor elements 53 and thin metal wires 54 are transfer-molded so as to be completely overcoated.
After that, as shown in FIG. 15B, connecting means 56 such as solder and a soldering ball is provided, wherein spherical solder 56 deposited to the bonding pad 51A is formed via the opening 52 by passing through a solder reflow furnace. Further, since semiconductor elements 53 are formed in the form of a matrix on the flexible sheet 50, these are diced to be separated from each other as shown in FIG. 14.
In addition, the sectional view of FIG. 15C shows electrodes 51A and 51D on both sides of the flexible sheet 50 as the electrodes. The flexible sheet 50 is generally supplied from a maker after both sides thereof are patterned.
Since a semiconductor apparatus that employs the above-described flexible sheet 50 does not utilize any publicly known metal frame, the semiconductor apparatus has a problem in that a multi-layer connection structure cannot be achieved while it has an advantage by which a remarkably thin package structure can be brought about, wherein path is carried out with one layer of copper foil pattern 51, which is provided substantially on the surface of the flexible sheet 50.
It is necessary to make the flexible sheet 50 sufficiently thick, for example, approx. 200 μm, in order to retain supporting strength to achieve a multi-layer connection structure. Therefore, there is a problem of retrogression with respect to thinning of the sheet.
Further, in the method for fabricating a circuit device, a flexible sheet 50 is transferred in the above-described fabrication apparatus, for example, a die bonder, wire bonder, a transfer mold apparatus, and a reflow furnace, etc., and the flexible sheet 50 is attached onto a portion called a “stage” or a “table”.
However, if the thickness of the insulation resin that becomes the base of the flexible sheet 50 is made thin, for example, 50 μm, the flexible sheet 50 may be warped as shown in FIG. 16 or its transfer performance may be remarkably worsened where the thickness of the copper foil pattern 51 formed on the surface thereof is thin to be 9 through 35 μm. In addition, another problem arises in that the flexible sheet 50 is defectively attached to the above-described stage or table. This is because it is considered that the resin is warped since the insulation resin itself is very thin, and the resin is warped due to a difference in the thermal expansion coefficient between the copper foil pattern 51 and the insulation resin. Particularly, if a hard insulation material in which no glass cloth fibers are used as a core material is warped as shown in FIG. 16, there was a problem in that the insulation material could easily collapse due to compression from above.
Since the portion of the opening 52 is compressed from above when being molded, a force by which the periphery of the bonding pad 51A is warped upward is brought about, the adhesion of the bonding pad 51A is worsened.
Also, the resin material that constitutes a flexible sheet 50 has less flexibility, or if a filler to increase the thermal conductivity is blended, the flexible sheet 50 is made hard. In such a case, where bonding is carried out by a wire bonder, there may be a case where the bonded portion is cracked. Also, when performing transfer molding, there is a case where the portion with which a metal die is brought into contact is cracked. This remarkably occurs if any warping shown in FIG. 16 is provided.
Although the flexible sheet 50 described above is such a type that no electrode is formed on the rear side thereof, there are cases where an electrode 51D is formed on the rear side of the flexible sheet 50 as shown in FIG. 15C. At this time, since the electrode 51D is brought into contact with the above-described fabrication apparatus or is brought into contact with the transfer plane of transfer means between the fabrication apparatuses, another problem occurs in that damage and scratches arise on the rear side of the electrode 51D, wherein the electrode is established with such damage and scratches retained, the electrode 51 itself may be cracked due to application of heat later on.
Also, if an electrode 51D is provided on the rear side of the flexible sheet 50, a problem occurs in that, when carrying out transfer molding, no facial contact with the stage can be secured. In this case, if the flexible sheet 50 is composed of a hard material as described above, the electrode 51D becomes a fulcrum and the periphery of the electrode 51D is compressed downward, wherein the flexible sheet 50 is cracked.
The present inventor proposed use of an insulation resin sheet for which the first thin conductive layer and the second thick conductive layer are adhered by insulation resin.
However, although the first conductive layer, which is thin, is finely patterned in achieving a multi-layer connection structure, there is a problem in that the second conductive layer, which is thick, is not suitable for fine patterning.